Method of and apparatus for tracking position error control

ABSTRACT

There are disclosed a method of and an apparatus for controlling a track position error of a load such as the workpiece table of a multi-axis machine tool. A feed pulse signal for driving the load is added to a deviation of a present position signal detected of the load from a commanded position signal dependent on the polarity of the deviation. The deviation of the added feed pulse signal from the present position signal of the load is determined, and a drive signal for driving the load is derived from a deviation signal indicative of the last-mentioned deviation. A drive source for driving the load is controlled with the deviation signal.

BACKGROUND OF THE INVENTION

The present invention relates to a method of and an apparatus fortracking position error control in a closed-loop servo system, and moreparticularly to a method of and an apparatus for controlling a trackingposition error so as to be of a desired value in a positioning servosystem for a machine tool or the like.

Prior positioning servo systems for use with machine tools or the likecontrol the position of an object, such as a workpiece table, with thedifference between feed command pulses and position feedback pulseswhich is applied as an actual speed command. The track acutally followedby the object is delayed at all times with respect to the actuallycommanded position, the delay varying dependent on the commanded feedspeed and the gain of the closed servo loop.

Ordinary digital servo systems comprise a differential counter forcounting feed command pulses and position feedback pulses and issuingthe difference as an output signal indicating a speed command, and a D/Aconverter for converting the output signal from the differential counterto an analog signal. The output signal of the differential counter isrepresentative of the delay of the load shaft position from thecommanded position, and depends on the commanded feed speed and theclosed-loop gain.

When a workpiece is to be machined along a given path by controlling twoor more axes in a machine tool, the contour to which the workpiece ismachined is subjected to an error if the gains of the closed servo loopsfor the respective axes about which the workpiece is displaced aredifferent from each other. To avoid this drawback, the controls forconventional machine tools have adjustable axis loop gains for achievinga desired degree of machining accuracy. However, when any contouringerror is to remain in the range of submicrons, the desired machiningaccuracy could not be accomplished only by adjusting the loop gains forthe respective axes. Particularly, it has been found in ultra-precisionmachine tools that the contouring error is produced by different delayscaused with respect to the respective axes when the loop gains in thecontrol servo system are different from axis to axis even if the samecommand speed is given to the axes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a track positionerror control method for a servo system employing a plurality of axeswith different loop gains, the method being capable of controlling thetrack error for each axis so as to fall within a predetermined range,preferably zero, so that any contouring error of a machine tool whichwould result from the different loop gains can be eliminated.

Another object of the present invention is to provide a method ofcontrolling a track position error of a load, comprising the steps ofadding a feed pulse signal for driving the load to a deviation of apresent position signal detected of the load from a commanded positionsignal dependent on the polarity of the deviation, determining adeviation of the added feed pulse signal from the present positionsignal of the load, deriving a drive signal for driving the load from adeviation signal indicative of the last-mentioned deviation, andcontrolling a drive source for driving the load with the deviationsignal.

Still another object of the present invention is to provide an apparatusfor controlling a track position error of a load, comprising a feedpulse generator for generating feed pulses to drive the load, a feedbackpulse generator for generating feedback pulses indicative of asubstantial driven condition of the load, a deviation detector fordetecting a deviation of the feedback pulses from the feed pulses, afirst D/A converter for converting a digital signal produced as adeviation signal from the deviation detector to an analog signal, acommand position counter for counting the feed pulses from the feedpulse generator, a load position counter for counting the feedback pulsegenerator, a processor for computing a digital signal indicative of thedifference between an output signal from the command position counterand an output signal from the load position counter, a second D/Aconverter for converting the digital signal from the processor to ananalog signal, and means for controlling the load with the analogsignals produced from the first and second D/A converters.

A still further object of the present invention is to provide anapparatus for controlling a track position error of a load, comprisingcorrective-pulse generator means for generating pulses of a frequencycorresponding to the difference between the count of a command positioncounter for counting feed pulses and the count of a load positioncounter for counting feedback pulses corresponding to the speed ofmovement of the load, and for generating a polarity signal representingthe polarity of the difference, an adder for selectively increasing andreducing the feed pulses with the pulses produced from thecorrective-pulse generator means dependent on the polarity, and meansfor driving the load according to the difference between a count ofoutput pulses from the adder and a count of the feedback pulses.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 a block diagram of an apparatus employed to carry out a method ofcontrolling a track position error according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an apparatus or electric circuit for carrying out atrack position error control method according to the present inventioncomprises a corrective-pulse generator 10 including a command positioncounter 12 for counting speed command pulses, i.e., feed pulses producedfrom a feed pulse generator 11, a load position counter 14 for countingposition feedback pulses, and a processor 16 for computing thedifference between the counts from the command position counter 12 andthe load position counter 14. The output terminal of the processor 16 isconnected to a correction counter 20 in which the output signal from theprocessor 16 is preset in response to a timing pulse supplied as a loadpulse from a timing pulse generator 18. The correction counter 20 has anoutput terminal coupled to a zero detector 22 which detects when thecount from the correction counter 20 falls to zero. The correctioncounter 20 also has another output terminal connected to an inputterminal of an adder 30 (described later). The output terminal of theprocessor 16 is also connected to a D/A converter 24 which converts theoutput signal from the processor 16 to an analog voltage. The D/Aconverter 24 has its output terminal joined to a V/F converter 26 forconverting the analog voltage from the D/A converter 24 to acorresponding frequency. The output terminal of the V/F converter 26 iscoupled to a gate circuit 28 which is closed in response to the outputsignal from the zero detector 22. The output signal from the gatecircuit 28 is supplied to the correction counter 20 and also to theadder 30. The adder 30 has another input terminal supplied with the feedpulses from the feed pulse generator 11.

The output terminal of the adder 30 is connected to an input terminal ofa deviation detector 32, with its other input terminal supplied with theposition feedback pulses that are also fed to the load position counter14. The output terminal of the deviation detector 32 is connected to aD/A converter 34 having its output terminal coupled to an analogsubtracter 36. The analog subtracter 36 has its output terminalconnected to an amplifier 38 with its output terminal coupled to a motor40. The rotatable drive axis of the motor 40 is mechanically coupled toa feed screw 42 held in threaded engagement with a workpiece table 44.The workpiece table 44 engages a pulse generator 46 comprising a rotaryencoder, a linear position detector, or the like. The output signal fromthe pulse. generator 46 is applied as the feed pulses to the deviationdetector 32 and the load position counter 14, as described above. To themotor 40, there is coaxially connected a tachogenerator 48 with itsoutput terminal connected to another input terminal of the analogsubtracter 36.

Operation of the electric circuit for effecting the track position errorcontrol method of the invention is as follows:

Feed pulses serving as a speed command to move the workpiece table 44are supplied from the feed pulse generator 11 to the adder 30 and thecommand position counter 12. The command position counter 12 counts thesupplied feed pulses, i.e., speed command pulses. The pulse generator 46detects, as a pulse train, the displacement of the workpiece table 44caused by the feed pulses and supplies such a pulse train of positionfeedback pulses to the load position counter 14. The load positioncounter 14 counts the supplied position feedback pulses. The count ofthe command position counter 12 is of a value corresponding to thecommanded position for the workpiece table 44. The count of the loadposition counter 14 is of a value corresponding to the actualdisplacement of the workpiece table 44. The difference ε between thecount from the command position counter 12 and the count from the loadposition counter 14 is computed by the processor 16. The output signal εfrom the processor 16 is preset in the correction counter 20 in responseto a timing pulse generated and applied by the timing pulse generator 18as a load signal to the correction counter 20. The processor 16 alsoapplies a signal indicative of the polarity of the differential signal εas well as the differential signal itself to the correction counter 20.The difference ε computed by the processor 16 is converted by the D/Aconverter 24 to a corresponding analog voltage, which is in turnconverterd by the V/F converter 26 to a corresponding frequency.Therefore, the frequency of output pulses from the V/F converter 26 iscommensurate with the computed difference ε from the processor 16.

When the computed difference ε from the processor 16 is not zero, thegate circuit 28 is open by the output signal fed from the correctioncounter 20 via the zero detector 22. Therefore, the output signal fromthe V/F converter 26 is introduced to the adder 30 through the gatecircuit 28. The output signal from the V/F converter 26 is also suppliedas corrective pulses via the gate circuit 28 to the correction counter20 to increase or reduce the count of the correction counter 20 to zero.Specifically, each time a corrective pulse from the V/F converter 26 isapplied, the count of the correction counter 20 is incremented ordecremented by 1 until the count of the correction counter 20 falls tozero. Consequently, the corrective-pulse generator 10 produces a numberof corrective pulses corresponding to the difference between thecommanded position and the actual load position, i.e., the accumulatedvalue representing the displacement of the workpiece table 44.

The corrective pulses generated by the corrective-pulse generator 10 areapplied to the adder 30, which is also fed with the polarity sign signalfrom the correction counter 20 in the corrective-pulse generator 10. Theadder 30 adds the supplied corrective pulses to or subtracts them fromthe feed pulses from the feed pulse generator 11 dependent on the signof the polarity sign signal supplied from the correction counter 20. Asa result, the corrected output speed command signals from the adder 30comprises a train of pulses produced by adding the corrective pulses orsubtracting them from the original speed command pulses supplied by thefeed pulse generator 11 dependent on the polarity sign from thecorrection counter 20.

The corrected feed pulses from the adder 30 and the position feedbackpulses from the pulse generator 46 are supplied to the deviationdetector 32 which then detects the deviation between the supplied pulsesignals. The deviation data thus detected by the deviation detector 32is temporarily latched therein and then converted by the D/A converter34 to a corresponding analog voltage which is amplified by the amplifier38. The amplified analog voltage from the amplifier 38 is applied to themotor 40 to drive same. The motor 40 rotates the feed screw 42 to movethe workpiece table 44, the displacemnt of which is detected by thepulse generator 46. The rotational speed of the motor 40 is detected bythe tachogenerator 48 coupled to the motor 40. The output signal fromthe tachogenerator 48 is fed back to the analog subtracter 36, so thatthe position of the workpiece table 44 can also be controlled by thisminor feeback loop.

As described above, a number of feedback pulses corresponding to thedisplacement or speed of movement of the workpiece table 44 aregenerated by the pulse generator 46, and the count of the load positioncounter 14 which counts the feedback pulses is commensurate with theload position, i.e., the workpiece table position.

The feedback loop following the deviation detector 32 is of conventionalnature. However, the feed pulses applied to the deviation detector 32are corrected by the corrective pulses according to the presentinvention. More specifically, the commanded speed is controlled by thepositional error between the commanded position and the actual loadposition so that the deviation of the actual load position from at leastthe commanded position will be equal to "zero" at all times. Therefore,the commanded speed is equivalently varied by the difference between thecommanded position and the actual load position, and the delay of thepresent load position from the commanded position can be controlled soas to be "zero" at all times. Since the delay can be eliminated for eachaxis, the contouring accuracy of a multi-axis machine tool can beincreased without requiring precise adjustment of the loop gains of therespective servo systems with respect to each other.

In the illustrated embodiment, the output signal from the processor 16is applied to the D/A converter 24, the output voltage signal of whichis converted by the V/F converter 26 to the corresponding frequency, andthe frequency signal from the V/F converter 26 is fed via the adder 30and the deviation detector 32 to the D/A converter 34. However, theoutput signal from the D/A converter 24 may be applied directly to theanalog subtracter 36 connected to the output terminal of the D/Aconverter 34. This can simplify the circuit arrangement, so that theapparatus can be manufactured less costly.

With the present invention, as described above, the feed pulses arecorrected by the positional error between the commanded position and theactual load position, and the displacement of the load is controlled bythe corrected feed pulses. As a consequence, the contouring accuracy ofa multi-axis machine tool is improved by the servo system of the presentinvention, which can be employed for machining operations requiring ahigh degree of accuracy in the range of submicrons. Specifically, thecontouring accuracy of a machine tool with two or more axes can beincreased without necessitating a complex adjustment process forbringing the loop gains of the respective axes into conformity with eachother.

The apparatus of the present invention is illustrated as beingimplemented by individual hardware components. However, the illustratedhardware mechanism may partly or entirely be replaced with a systemincluding a computer, and the respective component functions may beperformed on a time-division multiplexing basis.

Although a certain preferred embodiment has been shown and described, itshould be understood that many changes and modifications may be madetherein without departing from the scope of the appended claims.

What is claimed is:
 1. A method of controlling a track position error ofa load, comprising the steps of:adding a feed pulse signal for drivingsaid load to a deviation of a present position signal detected of saidload from a commanded position signal dependent on the polarity of saiddeviation; determining a first deviation of the feed pulse signal fromthe present position signal of said load; deriving a drive signal fordriving said load from a deviation signal indicative of saidlast-mentioned deviation; controlling a drive source for driving saidload with said deviation signal; determining a second diviation betweensaid drive signal for driving said load and a feedback signal producedby said drive source; and driving said drive source substantially with asignal representative of said second deviation.
 2. An apparatus forcontrolling a track position error of a load, comprising:a feed pulsegenerator for generating feed pulses to drive said load; a feedbackpulse generator for generating feedback pulses indicative of asubstantial driven condition of said load; a deviation detector fordetecting a deviation of said feedback pulses from said feed pulses; afirst D/A converter for converting a digital signal produced as adeviation signal from said deviation detector to an analog signal; acommand position counter for counting the feed pulses from said feedpulse generator; a load position counter for counting the feedback pulsegenerator; a processor for computing a digital signal indicative of thedifference between an output signal from said command position counterand an output signal from said load position counter; a second D/Aconverter for converting the digital signal from said processor to ananalog signal; and means for controlling said load with said analogsignals produced from said first and second D/A converters.
 3. Anapparatus for controlling a track position error of a load,comprising:corrective-pulse generator means for generating pulses of afrequency corresponding to the difference between the count of a commandposition counter for counting feed pulses and the count of a loadposition counter for counting feedback pulses corresponding to the speedof movement of said load, and for generating a polarity signalrepresenting the polarity of said difference; an adder for selectivelyincreasing and reducing said feed pulses with the pulses produced fromsaid corrective-pulse generator means dependent on said polarity; andmeans for driving said load according to the difference between a countof output pulses from said adder and a count of the feedback pulses. 4.An apparatus according to claim 3, wherein said corrective-pulsegenerator means comprises a processor for computing the differencebetween the counts from said command position and said load positioncounter, a correction counter connected to an output terminal of saidprocessor, a D/A converter connected to the output terminal of saidprocessor, a V/F conveter connected to an output terminal of said D/Aconverter, a gate circuit connected to an output terminal of said V/Fconverter, and a zero detector connected to an output terminal of saidcorrection counter for controlling said gate circuit, the arrangementbeing such that, when said gate circuit is closed by said zero detector,an output signal from said V/F converter is applied to said adder forselectively increasing and reducing the feed pulses and also to saidcorrection counter as clock pulses for selectively increasing andreducing the count of said correction counter to zero.
 5. An apparatusaccording to claim 4, further comprising a deviation detector connectedto an output terminal of said adder for detecting a deviation betweenthe feed pulses corrected by said adder and said feedback pulses, and aD/A converter for producing an analog drive signal for driving said loadin response to a digital output signal from said deviation detector. 6.An apparatus according to claim 5, further comprising a drive sourceresponsive to said drive signal for driving said load, said drive sourceincluding load displacement detector means for producing an outputsignal representing a displacement of said load, and an analogsubtracter for generating a deviation signal indicating the deviationbetween said output signal from said load displacement detector meansand said analog drive signal supplied from said last-mentioned D/Aconverter.